The present invention relates to a distributed feedback type semiconductor laser that is capable of oscillating in a single longitudinal mode by changing the gain of the laser resonator in the same direction as the propagation of the laser light at almost the same interval as the laser light wavelength.
It has been verified recently that a distributed feedback type semiconductor laser device whose angle of refraction is changed in the same direction as the laser beam propagation at almost the same interval as the laser light wavelength is effective for realizing single longitudinal mode oscillation. The distributed feedback type semiconductor laser element, having its angle of refraction changed, provides two longitudinal modes of basically equal gain. However, in a steady state operation where constant current is allowed to flow at all times, oscillation is assumed to take place in either of the two longitudinal modes due to an inherent asymmetry of the resonator. In most distributed feedback type semiconductor laser elements, one of the two cleavage planes is etched obliquely to prevent the two cleavage planes from constituting a Fabry-Perot resonator. Therefore, the asymmetry of the resonator increases significantly and a stable single longitudinal mode oscillation is realized.
However, when the above conventional distributed feedback type semiconductor laser element is used as a light source for optical communication, it is operated in a non-steady state with the amount of exciting current modulating rapidly. In such a non-steady state operation, the asymmetry of the resonator is considered to change continuously so that the laser element does not always oscillate in a single mode. Actually, oscillation spectrum expansions and discontinuous oscillation mode changes have been observed.